short story...buying pressure so MMs raise the ask before open to anything above yesterdays close and take your money there....then they usually come back down and "fill" the gap...or if you look on a chart you see a candle that covers part of the day befores candle...If they don't fill it, it remains a "gap" until they fill it...
Yes it is the difference in where the pps (Price per Share) ended yesterday and where it starts today. Naturally and normally it starts where it left off, but on days with buying pressure like today and yesterday they have no choice but to raise the pps.
Market Data 
Markets 
